1. Field of the Invention
This invention relates to heat sinks for cooling fluids suitable for operation in high altitude environment situations such as outer space.
2. Prior Art
Many attempts for providing heat sink devices for use in conjunction with a variety of environments are known. These devices find utilization in many diverse areas: cooling towers, air conditioning, space suits, and the like. In general, the use of a cooling fluid or source of low pressure is utilized to stimulate heat transfer. A most basic technique merely uses the passage of air to create a surface where evaporation may occur to facilitate cooling of a liquid. Other basic techniques rely on sublimation as the thermodynamic effect cooling the liquid. In practical application, heat sinks for cooling fluids may take the form of many different configurations. These devices can generally be categorized as metal porous plate sublimators, foam sublimators, spraying flash evaporators, wick-fed water boilers and membrane evaporator/sublimators. The metal plate and foam sublimators both use flat plate sublimation surfaces, and in terms of size are relatively large for a given heat rejection rate. Also, both systems require a finite time from the start of the operation until the system reaches a full capacity operation. In contrast, the spraying flash evaporator has an extremely short start-up time, but it requires a large volume and control system with moving parts rather than being self-regulating as are the metal plate and foam sublimators.
The wick-fed water boiler technique requires active control of the evaporation chamber pressure which is difficult to achieve with a compact reliable system. The membrane evaporator/sublimator is the most similar of the prior art approaches to this invention and uses a number of small tubes made of a water-permeable membrane material. The water is pumped through the inside of the tubes while the tube exterior is exposed to a pressure below the water's saturation pressure. Water then migrates through the tube wall to the outer surface where it evaporates, cooling the water remaining inside the tube. The major disadvantage of this approach is that excess cooling, such as occurring during operation at low heat rejection rates or in case of failure of the backpressure control system, may cause freezing of the water remaining in the tubes. This situation can result in stoppage of water circulation and breaking of the tubes.
Within the patent literature, many of these concepts are described. U.S. Pat. No. 3,170,303, Rannenberg et al, shows a porous plate sublimator, and an improvement, U.S. Pat. No. 3,197,973, shows sublimation cooling through a porous walled tube with a vacuum applied to the outside surface of the tube. As shown in FIG. 2 of this patent, the sublimator has an outer wall made from a porous material such as sintered nickel or stainless steel. A series of helices are placed in the chamber to agitate the refrigerant for increased surface effect.
The mode of operation in the Rannenberg et al devices is with the initial operation starting with a solid plug of ice in the passage 52. Sublimation occurs with the application of heat, and, as a result, the ice layer is reduced. A flow of water, once it encounters the subcooled passage wall 50, freezes thereby forming a new plug of ice to renew the cycle.
U.S. Pat. No. 3,403,531 to Oesterheld discloses an evaporation device where liquid to be cooled flows over the outside of a stack of porous tubes and the liquid is absorbed by the tubes. A chimney device, adjacent to the tubes, draws air through the tubes to evaporate small amounts of liquid. The cooling effect, as a function of evaporation, then cools the liquid passing over the outer surfaces of the tube. Similarly, U.S. Pat. No. 3,079,765 to LeVantine discloses a space unit utilizing the evaporation principle to cool liquid absorbed by a network of tubes made from a sponge-like substance when the liquid is in communication with a source of low pressure.